insertPairUnique after deletion

Deleting a key replaces the key with an [`EMPTY_PAIR_64`](https://github.com/owensgroup/SlabHash/blob/master/src/concurrent_map/warp/delete.cuh#L71). Then the first deleted key will  be overwritten during [unique insertion](https://github.com/owensgroup/SlabHash/blob/master/src/concurrent_map/warp/insert.cuh#L131) without checking all the bucket keys.